Gas turbine engines are inherently designed to operate with high gas turbine temperatures. For maximum efficiency normal operation approaches the maximum allowable temperature of the materials. Temperatures above these limits will lead to permanent damage.
Fuel control systems deliver metered fuel to the combustor. While the pilot controls the fuel to obtain the desired power, the automatic control system includes a minimum flow commensurate with minimum normal operating conditions.
Axial flow compressors are subject to an operating condition commonly referred to as compressor stall. When the angle of attack of the blades is such that the air suddenly separates from the blades, the blades suddenly cease to impart velocity pressure to the air. This causes a sharp reduction in the output of the compressor producing a significantly reduced airflow. The automatic control system sets fuel flow at or near the minimum setting and continued introduction of the minimum fuel amount in conjunction with this reduced airflow leads to excessively high temperatures in the combustor and the turbine.
Various operating condition can cause the stall, for instance, ingestion of hot gases into the engine due to the firing of a rocket or gun from an aircraft, extreme aircraft maneuvers, or departures. It is known that the stall may sometimes be eliminated by reducing fuel flow to the combustor thereby decreasing the back pressure on the compressor. This can be done by the pilot if he has time to give attention to the problem. He quickly retards the power request which may decrease the fuel to a low level.
Schemes to automatically recover from a stall are known, for instance U.S. Pat. No. 3,540,214. Here a rapid fuel flow reduction is automatically achieved by dumping fuel from the supply line in times in the order of hundredths of a second. This is done in response to a rate of pressure change within the combustor. The system therefore operates very quickly and is a one shot try at recovering from the stall.
An aircraft encounters a multiplicity of operating conditions. This can vary from high altitude low velocity flight, to low altitude high velocity flight. In each of these situations different air densities and flow rates are encountered and the particular fuel reduction required to recover from compressor stall in one condition will not be suitable for all others. Engine testing has demonstrated that fuel flow reduction schemes can actually inhibit stall recovery in some compression systems. Accordingly any stall recovery system is not sufficiently reliable to perform under all circumstances.
Should the engine fail to recover from the stall and then overheat, a pilot can face a difficult decision when occupied by other matters. He must either shut down the engine or leave it operating and hope that the damage can be tolerated. One must also consider that excessive reduction in fuel flow, whether accomplished automatically or manually, will lead to flame out of the engine. This would require additional pilot effort.